Penicillus (alga)

Lore & Background

The genus Penicillus was established by Lamarck in 1813, with Penicillus capitatus as the type species. Synonyms listed for the genus include Coralliodendron Kützing, 1841 and Corallocephalus Kützing, 1843. Current marine taxonomic databases place Penicillus in the family Halimedaceae, tribe Udoteeae, although the genus has also been treated in Udoteaceae. Molecular work on udoteacean algae found that traditional morphology-based boundaries among genera including Penicillus, Udotea, and Chlorodesmis do not always correspond to monophyletic lineages.

Species of Penicillus form erect, calcified thalli that may reach about 15 cm in height. The thallus consists of a rhizoidal holdfast embedded in sediment, a stalk or stipe, and a globose, subglobose, or pear-shaped capitulum made up of numerous loose siphons. Like other bryopsidalean algae, Penicillus is siphonous or coenocytic, with large multinucleate tubular cells rather than a body divided into many small cellular compartments. The genus is strongly calcareous, depositing calcium carbonate, especially aragonite, and calcium oxalate crystals have also been reported in Penicillus, Rhipocephalus, and Udotea.

Penicillus species are marine algae of shallow tropical and subtropical waters, especially familiar from Caribbean seagrass beds and sand flats, where they may occur with other calcified green algae such as Halimeda and Udotea. The rhizoidal base anchors the alga in unconsolidated sediment. At least some species have broad distributions; P. capitatus is known from the western Atlantic and Caribbean, and recent molecular work confirmed its occurrence in the Azores, where the population was found to be closely related to western Atlantic material.

Reader's Guide

Penicillus contributes to carbonate sediment production in shallow-water tropical and subtropical systems by producing aragonite. Its brush-like capitulum can also function as habitat for small animals. In a study of P. capitatus in Puerto Rico, the abundance and species richness of associated crustaceans increased with algal size, leading the author to describe individual thalli as 'algal islands' for macrocrustaceans. Nutrient physiology has been studied in P. capitatus associated with subtropical seagrass meadows in Bermuda; enrichment experiments found that photosynthetic response increased significantly with nitrogen addition, indicating nitrogen limitation in the populations studied. The genus has been studied as a producer of carbonate sediment, and its calcium oxalate crystals have been noted in genera known for producing sedimentary aragonite needles. The taxonomic placement of Penicillus remains somewhat uncertain, as molecular work has shown that traditional morphology-based boundaries among genera including Penicillus, Udotea, and Chlorodesmis do not always correspond to monophyletic lineages.

Did You Know?

• Penicillus is commonly called shaving-brush algae because of its upright stalk and brush-like head.
• In a study in Puerto Rico, individual thalli of P. capitatus were described as 'algal islands' for macrocrustaceans.
• Molecular work confirmed the occurrence of P. capitatus in the Azores, closely related to western Atlantic material.